In a turbomachine, stages of variable-pitch vanes are mounted between rotor disks of a high-pressure compressor. These variable-pitch vanes are carried by the stator and can be adjusted in position about their axes in order to optimize the flow of gases into the engine of the turbomachine.
A variable-pitch vane comprises an axial cylindrical pivot at each of its ends, these pivots defining the axis of rotation of the vane. The radially external pivot, termed actuating pivot, is engaged in a cylindrical duct of a casing of the turbomachine and is connected by a connecting rod to an actuating ring operated by a ram or an electric motor. The rotation of the actuating ring is transmitted by the connecting rods to the external pivots of the vanes and causes them to turn about their axes.
The radially internal pivot, termed guide pivot, is engaged in a cylindrical housing of a sectorized internal ring. Each ring sector is maintained radially on vanes by at least two cylindrical pins engaged in axial orifices in the ring sector, these pins being parallel to one another and extending on either side of the pivot of a vane while passing into an external annular groove in this pivot. The cylindrical pins housed in the grooves in the pivots maintain the ring sectors radially on these pivots.
The ring is mounted at its internal periphery on a sectorized annular shroud comprising blocks of abradable material intended to cooperate with seal teeth provided on the rotor of the turbomachine in order to limit the passage of air in the axial direction inside the ring. The shroud comprises a radially external annular rib engaged in an annular channel opening toward the inside of the ring sectors in order to align these ring sectors with one another. The rib of the shroud is also engaged in transverse notches in the cylindrical pins in order to maintain these pins axially in the orifices in the ring sectors.
In operation, during the rotation of a variable-pitch vane about its axis, the internal pivot of this vane is caused to rub against the cylindrical pins situated on either side of this pivot, thus resulting in unwanted rotational movements of the pins about their axes. These unwanted movements combine with the vibrations of the engine and cause wear on the rib of the shroud as a result of the bottoms of the notches in the cylindrical pins rubbing and bearing against this rib. When the degree of wear on this rib has exceeded a certain threshold, it is required to replace the annular shroud, which is a long and costly operation.